1. Field of the Invention
This invention relates to knocking control systems in engines, and more particularly to a knocking control system capable of detecting knocking in an internal combustion engine and retarding ignition timing when knocking occurs.
2. Description of the Prior Art
The conventional knocking control system includes a vibration pickup for detecting engine vibrations; an output from this vibration pickup is full-wave rectified by a full-wave rectifier and thereafter integrated by an integrator, whereby a mean value is extracted. An output signal from this integrator is summed with a voltage of a predetermined value by an adder, whereby a reference voltage can be extracted. This reference voltage is inputted to a comparator, and the output signal from the vibration pickup is also inputted to the comparator, where the magnitudes of the output signal from the vibration pickup and the reference voltage are compared with each other. This knocking control system determines an occurrence of knocking when the output signal from the vibration pickup exceeds the reference voltage, whereupon the spark advance is controlled to lag through predetermined degrees of crankshaft angle.
However, in the conventional knocking control system, there has been a problem because the integration time of the integrator is as long as 400 msec, whereby the maximum value of the output from the integrator is lowered and there is a possibility that the maximum value of the background noise amplitude will exceed the reference voltage, so that an incorrect indication of knocking will be detected. In addition, when vibrations of a multi-cylinder engine are detected by a vibration pickup, vibrations due to knocking occurring in a cylinder remote from the vibration pickup are attenuated, while opening and closing sounds of valves near the vibration pickup are high in noise level so that if the added voltage is high enough to offset the background noise, when knocking occurs in a cylinder remote from the vibration pickup, the knocking cannot be detected.
To overcome the above problem, it has been proposed to preset the spark advance of a cylinder remote from the vibration pickup to lag several degrees so as to reduce the frequency of occurrences of knocking. However, this results in increased fuel consumption.